“The female chimpanzee at the Philadelphia Zoological Garden died of complications from a cold early in the morning of December 27, 1878. “Miss Chimpanzee,” according to news reports, died “while receiving the attentions of her companion.” Both she and that companion, a four-year-old male, had been born near the Gabon River, in West Africa; they had arrived in Philadelphia in April, together. “These Apes can be captured only when young,” the zoo superintendent, Arthur E. Brown, explained, and they are generally taken only one or two at a time. In the wild, “they live together in small bands of half a dozen and build platforms among the branches, out of boughs and leaves, on which they sleep.” But in Philadelphia, in the monkey house, where it was just the two of them, they had become “accustomed to sleep at night in each other’s arms on a blanket on the floor,” clutching each other, desperately, achingly, through the long, cold night.

The Philadelphia Zoological Garden was the first zoo in the United States. It opened in 1874, two years after Charles Darwin published “The Expression of the Emotions in Man and Animals,” in which he related what he had learned about the social attachments of primates from Abraham Bartlett, the superintendent of the Zoological Society of London:

Many kinds of monkeys, as I am assured by the keepers in the Zoological Gardens, delight in fondling and being fondled by each other, and by persons to whom they are attached. Mr. Bartlett has described to me the behavior of two chimpanzees, rather older animals than those generally imported into this country, when they were first brought together. They sat opposite, touching each other with their much protruded lips; and the one put his hand on the shoulder of the other. They then mutually folded each other in their arms. Afterwards they stood up, each with one arm on the shoulder of the other, lifted up their heads, opened their mouths, and yelled with delight.

Mr. and Miss Chimpanzee, in Philadelphia, were two of only four chimpanzees in America, and when she died human observers mourned her loss, but, above all, they remarked on the behavior of her companion. For a long time, they reported, he tried in vain to rouse her. Then he “went into a frenzy of grief.” This paroxysm accorded entirely with what Darwin had described in humans: “Persons suffering from excessive grief often seek relief by violent and almost frantic movements.” The bereaved chimpanzee began to pull out the hair from his head. He wailed, making a sound the zookeeper had never heard before: Hah-ah-ah-ah-ah. “His cries were heard over the entire garden. He dashed himself against the bars of the cage and butted his head upon the hard-wood bottom, and when this burst of grief was ended he poked his head under the straw in one corner and moaned as if his heart would break.”

Nothing quite like this had ever been recorded. Superintendent Brown prepared a scholarly article, “Grief in the Chimpanzee.” Even long after the death of the female, Brown reported, the male “invariably slept on a cross-beam at the top of the cage, returning to inherited habit, and showing, probably, that the apprehension of unseen dangers has been heightened by his sense of loneliness.”

Loneliness is grief, distended. People are primates, and even more sociable than chimpanzees. We hunger for intimacy. We wither without it. And yet, long before the present pandemic, with its forced isolation and social distancing, humans had begun building their own monkey houses. Before modern times, very few human beings lived alone. Slowly, beginning not much more than a century ago, that changed. In the United States, more than one in four people now lives alone; in some parts of the country, especially big cities, that percentage is much higher. You can live alone without being lonely, and you can be lonely without living alone, but the two are closely tied together, which makes lockdowns, sheltering in place, that much harder to bear. Loneliness, it seems unnecessary to say, is terrible for your health. In 2017 and 2018, the former U.S. Surgeon General Vivek H. Murthy declared an “epidemic of loneliness,” and the U.K. appointed a Minister of Loneliness. To diagnose this condition, doctors at U.C.L.A. devised a Loneliness Scale. Do you often, sometimes, rarely, or never feel these ways?

I am unhappy doing so many things alone.I have nobody to talk to.I cannot tolerate being so alone.I feel as if nobody really understands me.I am no longer close to anyone.There is no one I can turn to.I feel isolated from others.

In the age of quarantine, does one disease produce another?

“Loneliness” is a vogue term, and like all vogue terms it’s a cover for all sorts of things most people would rather not name and have no idea how to fix. Plenty of people like to be alone. I myself love to be alone. But solitude and seclusion, which are the things I love, are different from loneliness, which is a thing I hate. Loneliness is a state of profound distress. Neuroscientists identify loneliness as a state of hypervigilance whose origins lie among our primate ancestors and in our own hunter-gatherer past.

Much of the research in this field was led by John Cacioppo, at the Center for Cognitive and Social Neuroscience, at the University of Chicago. Cacioppo, who died in 2018, was known as Dr. Loneliness. In the new book “Together: The Healing Power of Human Connection in a Sometimes Lonely World” (Harper Wave), Murthy explains how Cacioppo’s evolutionary theory of loneliness has been tested by anthropologists at the University of Oxford, who have traced its origins back fifty-two million years, to the very first primates. Primates need to belong to an intimate social group, a family or a band, in order to survive; this is especially true for humans (humans you don’t know might very well kill you, which is a problem not shared by most other primates). Separated from the group—either finding yourself alone or finding yourself among a group of people who do not know and understand you—triggers a fight-or-flight response. Cacioppo argued that your body understands being alone, or being with strangers, as an emergency. “Over millennia, this hypervigilance in response to isolation became embedded in our nervous system to produce the anxiety we associate with loneliness,” Murthy writes. We breathe fast, our heart races, our blood pressure rises, we don’t sleep. We act fearful, defensive, and self-involved, all of which drive away people who might actually want to help, and tend to stop lonely people from doing what would benefit them most: reaching out to others.

The loneliness epidemic, in this sense, is rather like the obesity epidemic. Evolutionarily speaking, panicking while being alone, like finding high-calorie foods irresistible, is highly adaptive, but, more recently, in a world where laws (mostly) prevent us from killing one another, we need to work with strangers every day, and the problem is more likely to be too much high-calorie food rather than too little. These drives backfire.

Loneliness, Murthy argues, lies behind a host of problems—anxiety, violence, trauma, crime, suicide, depression, political apathy, and even political polarization. Murthy writes with compassion, but his everything-can-be-reduced-to-loneliness argument is hard to swallow, not least because much of what he has to say about loneliness was said about homelessness in the nineteen-eighties, when “homelessness” was the vogue term—a word somehow easier to say than “poverty”—and saying it didn’t help. (Since then, the number of homeless Americans has increased.) Curiously, Murthy often conflates the two, explaining loneliness as feeling homeless. To belong is to feel at home. “To be at home is to be known,” he writes. Home can be anywhere. Human societies are so intricate that people have meaningful, intimate ties of all kinds, with all sorts of groups of other people, even across distances. You can feel at home with friends, or at work, or in a college dining hall, or at church, or in Yankee Stadium, or at your neighborhood bar. Loneliness is the feeling that no place is home. “In community after community,” Murthy writes, “I met lonely people who felt homeless even though they had a roof over their heads.” Maybe what people experiencing loneliness and people experiencing homelessness both need are homes with other humans who love them and need them, and to know they are needed by them in societies that care about them. That’s not a policy agenda. That’s an indictment of modern life.

In “A Biography of Loneliness: The History of an Emotion” (Oxford), the British historian Fay Bound Alberti defines loneliness as “a conscious, cognitive feeling of estrangement or social separation from meaningful others,” and she objects to the idea that it’s universal, transhistorical, and the source of all that ails us. She argues that the condition really didn’t exist before the nineteenth century, at least not in a chronic form. It’s not that people—widows and widowers, in particular, and the very poor, the sick, and the outcast—weren’t lonely; it’s that, since it wasn’t possible to survive without living among other people, and without being bonded to other people, by ties of affection and loyalty and obligation, loneliness was a passing experience. Monarchs probably were lonely, chronically. (Hey, it’s lonely at the top!) But, for most ordinary people, daily living involved such intricate webs of dependence and exchange—and shared shelter—that to be chronically or desperately lonely was to be dying. The word “loneliness” very seldom appears in English before about 1800. Robinson Crusoe was alone, but never lonely. One exception is “Hamlet”: Ophelia suffers from “loneliness”; then she drowns herself.

Modern loneliness, in Alberti’s view, is the child of capitalism and secularism. “Many of the divisions and hierarchies that have developed since the eighteenth century—between self and world, individual and community, public and private—have been naturalized through the politics and philosophy of individualism,” she writes. “Is it any coincidence that a language of loneliness emerged at the same time?” It is not a coincidence. The rise of privacy, itself a product of market capitalism—privacy being something that you buy—is a driver of loneliness. So is individualism, which you also have to pay for.

Alberti’s book is a cultural history (she offers an anodyne reading of “Wuthering Heights,” for instance, and another of the letters of Sylvia Plath). But the social history is more interesting, and there the scholarship demonstrates that whatever epidemic of loneliness can be said to exist is very closely associated with living alone. Whether living alone makes people lonely or whether people live alone because they’re lonely might seem to be harder to say, but the preponderance of the evidence supports the former: it is the force of history, not the exertion of choice, that leads people to live alone. This is a problem for people trying to fight an epidemic of loneliness, because the force of history is relentless.

Before the twentieth century, according to the best longitudinal demographic studies, about five per cent of all households (or about one per cent of the world population) consisted of just one person. That figure began rising around 1910, driven by urbanization, the decline of live-in servants, a declining birth rate, and the replacement of the traditional, multigenerational family with the nuclear family. By the time David Riesman published “The Lonely Crowd,” in 1950, nine per cent of all households consisted of a single person. In 1959, psychiatry discovered loneliness, in a subtle essay by the German analyst Frieda Fromm-Reichmann. “Loneliness seems to be such a painful, frightening experience that people will do practically everything to avoid it,” she wrote. She, too, shrank in horror from its contemplation. “The longing for interpersonal intimacy stays with every human being from infancy through life,” she wrote, “and there is no human being who is not threatened by its loss.” People who are not lonely are so terrified of loneliness that they shun the lonely, afraid that the condition might be contagious. And people who are lonely are themselves so horrified by what they are experiencing that they become secretive and self-obsessed—“it produces the sad conviction that nobody else has experienced or ever will sense what they are experiencing or have experienced,” Fromm-Reichmann wrote. One tragedy of loneliness is that lonely people can’t see that lots of people feel the same way they do.

“During the past half century, our species has embarked on a remarkable social experiment,” the sociologist Eric Klinenberg wrote in “Going Solo: The Extraordinary Rise and Surprising Appeal of Living Alone,” from 2012. “For the first time in human history, great numbers of people—at all ages, in all places, of every political persuasion—have begun settling down as singletons.” Klinenberg considers this to be, in large part, a triumph; more plausibly, it is a disaster. Beginning in the nineteen-sixties, the percentage of single-person households grew at a much steeper rate, driven by a high divorce rate, a still-falling birth rate, and longer lifespans over all. (After the rise of the nuclear family, the old began to reside alone, with women typically outliving their husbands.) A medical literature on loneliness began to emerge in the nineteen-eighties, at the same time that policymakers became concerned with, and named, “homelessness,” which is a far more dire condition than being a single-person household: to be homeless is to be a household that does not hold a house. Cacioppo began his research in the nineteen-nineties, even as humans were building a network of computers, to connect us all. Klinenberg, who graduated from college in 1993, is particularly interested in people who chose to live alone right about then.

I suppose I was one of them. I tried living alone when I was twenty-five, because it seemed important to me, the way owning a piece of furniture that I did not find on the street seemed important to me, as a sign that I had come of age, could pay rent without subletting a sublet. I could afford to buy privacy, I might say now, but then I’m sure I would have said that I had become “my own person.” I lasted only two months. I didn’t like watching television alone, and also I didn’t have a television, and this, if not the golden age of television, was the golden age of “The Simpsons,” so I started watching television with the person who lived in the apartment next door. I moved in with him, and then I married him.

This experience might not fit so well into the story Klinenberg tells; he argues that networked technologies of communication, beginning with the telephone’s widespread adoption, in the nineteen-fifties, helped make living alone possible. Radio, television, Internet, social media: we can feel at home online. Or not. Robert Putnam’s influential book about the decline of American community ties, “Bowling Alone,” came out in 2000, four years before the launch of Facebook, which monetized loneliness. Some people say that the success of social media was a product of an epidemic of loneliness; some people say it was a contributor to it; some people say it’s the only remedy for it. Connect! Disconnect! The Economistdeclared loneliness to be “the leprosy of the 21st century.” The epidemic only grew.

This is not a peculiarly American phenomenon. Living alone, while common in the United States, is more common in many other parts of the world, including Scandinavia, Japan, Germany, France, the U.K., Australia, and Canada, and it’s on the rise in China, India, and Brazil. Living alone works best in nations with strong social supports. It works worst in places like the United States. It is best to have not only an Internet but a social safety net.

Then the great, global confinement began: enforced isolation, social distancing, shutdowns, lockdowns, a human but inhuman zoological garden. Zoom is better than nothing. But for how long? And what about the moment your connection crashes: the panic, the last tie severed? It is a terrible, frightful experiment, a test of the human capacity to bear loneliness. Do you pull out your hair? Do you dash yourself against the walls of your cage? Do you, locked inside, thrash and cry and moan? Sometimes, rarely, or never? More today than yesterday? ♦

Jill Lepore is a staff writer at The New Yorker and a professor of history at Harvard University.